Depending on the project and your role in it, you
might get lots of different benefits.

Learn new languages and tools to keep your skill
set current.

Practice techniques that you might not be able
to justify putting time into in a corporate
environment. (For example, coding for extreme
security or efficiency or minimum power and memory
usage.)

Make connections with people outside your company.

Signal your technical competence and ability to
work with others. Often, willingness to put time
into open source depends on the job market for
high-skill non-management programmers. The more
that the hiring process depends on formal education
and certification, and the less input it has from
peers, the less incentive that a programmer has
to Signal his or her skill using open source.

Talk with real users about bugs and features
without a company filter, to get a better
understanding of a software problem space.